(1) Field of the Invention
The present invention generally relates to a control system for controlling a suspension apparatus for use in damping force control, attitude control or vehicle height control of a vehicle. More particularly, the present invention is directed to controlling the damping force of a shock absorber or the spring constant of a suspension.
(2) Description of the Related Art
There is known a control system for controlling a variable damping force type shock absorber (see Japanese Laid-Open Patent Application No. 63-6238). A damping force of a shock absorber provided for each wheel is controlled depending on the detected level of a damping force detecting sensor attached to a corresponding one of the wheels. That is, the damping forces of the shock absorbers are separately controlled. There is also known a control system for controlling the shock absorbers or air suspensions (see Japanese Laid-Open Patent Application No. 60-148710). According to this Japanese application, when the vehicle is in the braking state, it is repeatedly determined whether or not a change in attitude of the vehicle due to pitching exceeds a predetermined level. When the determination result is affirmative, the damping forces of the shock absorbers or the spring constants of the air suspensions are separately increased so that the occurrence of a nose dive and its reaction are suppressed.
However, the above-mentioned control systems have problems arising from the fact that the shock absorbers or the air suspensions are separately controlled. There is a possibility that the balance between the damping forces related to the right and left wheels will be broken due to a rough road surface so that a rolling of the vehicle takes place. This is especially apparent when the vehicle speed is low. For example, a rolling occurs when trying to suppress a nose dive in the braking state. A rolling of the vehicle occurs when trying to suppress a squat in the rapidly accelerating state. Also, the road holding (degree of contact between each wheel and road) may be deteriorated when the damping forces of the shock absorbers are separately controlled. This affects the safety of the vehicle. The deterioration of the road holding also takes place when the spring constants of the air suspensions are separately controlled.